1. Field of the Invention
The present invention relates to the field of ink compositions and varnishes and more particularly to ink varnishes and methods useful in the manufacture of ink compositions which exhibit desirable ink properties while at the same time substantially reducing or eliminating use of petroleum solvents. The present invention also relates to ink varnishes having functional components providing the varnishes and resulting ink compositions with desirable properties including ink set, life, gloss, water washability and pigment dispersion, among others, and to methods of making such ink varnishes including an improved varnish cook cycle which maintains such functional components substantially in their functional form during the cook cycle.
2. Description of the Prior Art
A wide variety of printing processes currently exist in the art. Although the ink composition varnish and method of the present invention have applicability to most if not all of these prior processes, it has particular applicability to a lithographic printing process commonly referred to as offset lithography.
Traditional ink compositions are comprised of an ink varnish and a dispersed pigment. Ink varnishes in turn include a resin component comprised of a hard resin, a liquid resin or a combination of hard and liquid resins, an oil component and a solvent component. The oil and solvent components are comprised of materials such as vegetable oils, vegetable oil derivatives or combinations thereof, petroleum distillates and a variety of replacement solvents. Various ink properties such as ink set, life, gloss, tack, viscosity and color development depend upon the ink varnish components and in particular the selection of the resin, oil and solvent components. Solvents in printing inks perform several functions. First, they provide solvency during the varnish manufacturing step by dissolving and carrying the various resins. Second, they are used during final adjustment of the ink varnish and composition to achieve the desired ink tack and viscosity. Third, solvents provide the primary means to set the ink while also controlling, to a major extent, the ink life. As used in the printing industry, the "set" of an ink reflects the time needed for a printed ink to set up via evaporation, reaction, absorption or the like to a point where it can handled without smearing of the ink. The "life" of an ink reflects the time during which the ink can remain on a print plate or other application equipment without setting or drying. Generally, the properties of ink set and ink life are balanced. For example, conventional inks which set quickly tend to have a short life on the press, while inks which have a long life on the press generally take longer to set, especially on coated papers or other less porous substrates.
The degree of solution among the traditional ink varnish components also significantly affects various ink properties. In general, as the degree of varnish resin solubility increases, ink stability and gloss increase while viscosity decreases. Because cooking of the varnish generally improved solubility of the conventional varnish components, conventional varnish cook cycles have favored long cook times at elevated temperatures. For example, conventional cook cycles involve cook temperatures as much as 100.degree. F. (38.degree. C.) or more above the melting point of the hard resins and cook times as long as ten hours or more.
Various attempts have been made to improve and optimize the various ink properties by developing new ink resins, by using different solvents and oils and by varying the ratios and using different combinations of resin, oils and solvents. The solvents of choice in traditional ink varnish formulation have included petroleum distillates because of their ability to readily dissolve the conventional ink resins, because of their high volatility and because of their relatively low molecular weight. The high volatility causes the solvents to evaporate quickly from the printed ink while the low molecular weight enables the petroleum distillates to readily absorb into the substrate immediately after printing. Such evaporation and absorption causes the viscosity of the ink film to increase rapidly, thereby causing the ink to set.
Thus, traditional ink varnishes have included petroleum distillates as solvents and have required cooking the varnish at elevated temperatures and for extended periods of time to improve resin solubility and thus stability. However, there are two principal disadvantages of using petroleum distillates and other petroleum based solvents. First, because of their high volatility, petroleum distillates emit volatile organic compounds (VOCs) both during printing and cleanup as well as during the setting and drying. Thus, significant restrictions have been imposed on their use by Occupational Safety and Health Agency (OSHA), Environmental Protection Agency (EPA) and other regulations. Secondly, petroleum distillates are essentially a nonrenewable resource.
Because of the desire to reduce the use of petroleum based solvents in ink compositions, ink compositions and varnishes have been developed in which all or a portion of the petroleum based solvents have been replaced with nonvolatile organic solvents. For example, in U.S. Pat. No. 5,173,113 issued to Sugarman, the petroleum solvents have been replaced by various unsaturated fatty acid esters. Further, in U.S. Pat. No. 4,938,801 issued to Yoshioka, a portion of the petroleum solvent has been replaced by a nonvolatile polar solvent which is then denatured. Attempts have also been made to substitute vegetable oils such as linseed oil, soy oil, canola oil or tung oil in printing inks in place of the petroleum distillates, however, the use of such vegetable oils has been found to be particularly restrictive because of the limited compatibility and solubility of most hard resins in such oils and the longer set times of inks made with these oils. Still further, as disclosed in U.S. Pat. No. 5,178,672 issued to Miller, various fatty acids and fatty acids esterified with a simple alcohol or glycol have been used.
Although the replacement of petroleum solvents with nonvolatile or less volatile solvents has achieved the primary objective of reducing dependence on petroleum based solvents, many of the ink properties, most notably ink set, formulation latitude and gloss, have suffered. Further, although the solvent formulation has changed, the methods of manufacturing the ink varnishes, and in particular the varnish cook cycles, have not changed significantly. For example, current ink varnish manufacturing technology, including technology relating to varnishes having a significant nonvolatile solvent component or other functional component, continues to involve cooking the varnish components at elevated temperatures well above the melting point of the hard resins and for an extended period of time in an effort to improve the solubility of the various varnish components and thus the varnish stability. However, as discussed below, such cook cycles result in esterifying reactions among the various varnish components and in particular the solvent replacements and/or the other functional components. These reactions result in the loss of many of the possible additional benefits of utilizing the nonvolatile solvents and various other functional varnish components.
Accordingly, there is a need in the art for an ink composition or varnish, and a method of making the same, in which the use of petroleum solvents has been significantly reduced or eliminated, but which continues to have highly acceptable stability, highly acceptable ink properties including ink set, life, gloss, tack, viscosity and pigment dispersion and which retains additional benefits of the nonvolatile replacement solvents. A need also exists for a method of making an ink composition or varnish including a cook cycle having a primary objective of dissolving the varnish components, while also limiting reactions among functional components, thereby retaining the beneficial functional properties of such components in the finished varnish.